Introduction

Anterior cruciate ligament (ACL) reconstructive surgery can be performed under different types of anesthesia and analgesia. Patients can choose between general anesthesia and regional anesthesia. Usually general anesthesia is chosen by patients that “do not want to feel or know anything” during the procedure. On the other hand, there are many patients who are terrified of general anesthesia, of “giving up control of themselves,” and prefer regional anesthesia. Regional anesthesia includes central neuraxial anesthesia—spinal, epidural, or combined spinal epidural anesthesia—and peripheral nerve blockade.

Central neuraxial anesthesia gained its popularity in orthopaedic surgery because it is associated with reduced blood loss and fewer thromboembolic complications. Lumbar epidural blockade offers good postoperative analgesia, but the increased risk of hypotension, urinary retention, pruritus, and serious neurological complications have always troubled patients, anesthesiologists, and orthopaedic surgeons. On top of that, when epidurals were compared with femoral nerve blocks (FNBs) for total knee arthroplasty, it was shown that FNB can provide just as good analgesia with fewer side effects.

Peripheral nerve blocks can be performed as the main anesthetic technique for ACL reconstructive surgery or as adjuncts for analgesia to general or spinal anesthesia. These techniques, with the additional development of ultrasound guidance, seem to offer intraoperative analgesia, efficient postoperative pain control, fast recovery, and fewer side effects, all resulting in higher patient satisfaction.

Today, in the context of the global financial crisis and its apparent effect on health economics, the issue of cost in perioperative care demands our attention. This is one of the reasons that the prevalence of ambulatory surgery is now on the rise. Studies have demonstrated that timely treatment of pain and postoperative nausea and vomiting allows patients to safely accelerate their discharge from the hospital. Peripheral nerve blocks in patients undergoing ACL reconstruction can lead to discharge from the hospital on the same day of the surgery and therefore reduce costs.

ACL reconstructive surgery can be performed under the combination of different peripheral nerve blocks. In order to achieve a painless procedure, the nerves that innervate the anatomic area and structures of the knee that are affected by the ACL surgery must be blocked. These nerves are the femoral, the obturator, and the sciatic nerve. The lateral femoral cutaneous provides only sensory innervations to the anterior lateral and posterior aspects of the thigh, and problems usually appear during ACL reconstruction surgery once the tourniquet time is prolonged Special attention has been given recently to the obturator nerve, as it provides sensory branches, such as the articular branch for the knee and the cutaneous branches of the medial thigh, which contribute to the sensation of pain during knee surgery. Traditionally, the combination used is the posterior lumbar plexus block (PLPB)—also known as psoas compartment block—with the sciatic nerve block. In the more recent years, with the assistance of ultrasound guidance, more peripheral blocks have been used, the combination of femoral, obturator, and sciatic nerve blocks.

The lumbar plexus is formed within the psoas muscle from the L1-4 nerve roots, while a contribution of the 12th thoracic nerve is common. From the different branches of the plexus, the nerves that are of importance for the ACL reconstruction surgery are the femoral, lateral femoral cutaneous, and obturator nerve. The patient is placed in the lateral position, hips flexed, with the side to be blocked placed uppermost. The surface landmarks are the spinous process of L4 and the posterior superior iliac spine. Different approaches to the lumbar plexus have been described. The approach by Winnie at the level of L4–L5 is probably the most popular one. It is worth noting that when using the PLPB, the obturator nerve has higher failure rates than the femoral nerve. The approach of the PLPB at the level of L3 gives somehow higher success rates for the obturator nerve, but the risk for kidney injury is also higher.

The PLPB has limited popularity due to its relatively high rates of failure and serious complications. The reported failure rate for PLPB under neurostimulation is 5%–7% and has been reported to reach up to 20%. The complications of this block are numerous and more serious, as this is more than a simple “peripheral nerve block” due to its proximity to the epidural space, the ovarian/testicular vessels, the ureter, the retroperitoneal space, and the lower pole of the kidney. Thus on top of the complications that are expected in every peripheral nerve block, in this block total spinal anesthesia, epidural spread of the local anesthetic, renal hematoma, or pneumocele may happen. The use of ultrasound guidance could reduce the incidence of the complications mentioned previously. However, scanning of the lumbar plexus and directing the needle in real time under ultrasound guidance can be very challenging. The depth of the nerve roots and the “acoustic shadow” of the transverse processes are the main factors that make direct imaging technically difficult to achieve.

For all these reasons mentioned previously, anesthesiologists turned to more peripheral nerve blocks. Instead of blocking all three nerves centrally, the thought of blocking them independently was born. The femoral, lateral femoral cutaneous, and obturator nerve can be blocked with the patient in the supine position, peripherally, with higher success rates and less complications. The use of ultrasound guidance alone or in combination with neurostimulation is much easier and more efficient at this level.

The femoral nerve block should not be confused with the anterior lumbar plexus block or the so-called “3 in 1” block. The “3 in 1” as described by Winnie et al usually fails to block the obturator nerve. The femoral nerve can be blocked in the supine position below the inguinal ligament level, where it is superficial laterally to the femoral artery. For ultrasound guidance, a high-frequency linear probe is required. The nerve is identified in a transverse section, as a triangular hyperechoic region which lies lateral to the artery, deep to the fascia lata and to the fascia iliaca. Ultrasound guidance for FNB allows faster onset, increases the quality of sensory and motor block, reduces local anesthetic requirements, and has a lower risk for accidental vascular puncture. In addition, FNB can be used either as single-shot or as continuous nerve blockade via a catheter in combination with general or spinal anesthesia in order to offer quality pain control for ACL reconstructive surgery.

The obturator nerve has a great deal of anatomic variations, as confirmed by cadaveric and sonographic studies. The common obturator nerve gives the anterior and posterior rami, providing articular and cutaneous sensory nerves for the knee and the medial thigh. Ultrasound guidance is necessary in order to achieve high success rates for this block. The sonographic visualization of obturator nerve may not be easy due to the fact that it is small in size with a small deep course in the proximal thigh, and it lies between muscles. Different approaches have been proposed for the ultrasound-guided obturator nerve block.

Obturator Nerve Block Technique

In our institution the obturator nerve block is performed under dual guidance, with patients in supine position with the thigh of the affected leg slightly externally rotated. The inguinal ligament and the adductor longus are palpated. The ultrasound probe is positioned opposite the angle formed by the inguinal crease and adductor longus, with its short diameter approximately 2.5–3.0 cm in the course of the dichotomous of the angle. It is positioned perpendicular to the skin, and the area is scanned with small changes of the angle. In this position, the pectineus muscle, the adductor longus, the adductor brevis, and part of the adductor magnus muscles are visible, and the anterior division of the obturator nerve may be recognizable between adductor longus and adductor brevis, while the posterior division may be seen below the adductor brevis. In such cases, tracing the anterior ramus proximally toward the nerve, in the sonographic triangle formed by pectineus, adductor longus, and adductor brevis muscles, the ultrasound probe is tilted cephalically and the obturator nerve is apparent just as it bifurcates. In this plane, next to pectineus and below adductor longus, usually in a depth of approximately 2.0–2.5 cm, a “spider net” image is visualized. To proceed with the local anesthetic administration, it is mandatory that adductor longus, brevis, and magnus, all the three muscles, are contracting. The success of the block is assessed by evaluating only the motor function. The assessment consists of a request to the patient to carry out a simple movement, which includes asking the patient to lift his or her leg. In a successful obturator block, the patient could not lift the leg directly upwards, but he or she lifted it diagonally outwards (upwards and laterally). Then the patient is asked to leave down the leg. As a result of a successful block, the patient put the leg down laterally and could not adduct it to medial line. In case of branch block, lifting and leaving down the leg remains unaffected.